1. Field of the Invention
The present invention relates to a method of preparing shaped objects from a thermally expandable foam.
2. Background of the Prior Art
One technique for preparing molded objects of various shapes is known as the thermal expansion molding process (TEM). This method has the unique feature of providing pressure necessary for forming finished shapes by use of a thermally expandable (non-reversible) material placed within the female cavity of a mold. In this process, a mandrel (of a thermally expandable material such as silicone rubber) is wrapped with a resin preimpregnated fabric material. This wrapped mandrel is then inserted into a closed cavity mold. Then, the mandrel, mold or both mandrel and mold are heated with the result that the material of the mandrel expands thereby causing the material to expand against the female cavity mold. Pressure is created within the cavity by the confining surfaces of the mold. Significant pressures can be generated by this process which leads to high quality molded objects, even of complex shapes which are free of voids.
The conventional TEM process has several advantages over vacuum bag and autoclave curing of objects. An important advantage is that the TEM process obviates the need for the capital investment required to support autoclaves. Moreover, significant labor savings can be realized by the fact that the expensive bagging and debagging operations associated with autoclave processes are not necessary. Still further, the rate of rejection of product is reduced because of the above operational advantages, since bag leaks are a significant problem of autoclave curing operations. TEM processing is especially well suited in manufacturing parts of complex shapes because it is especially difficult to bag parts of complex shapes.
Disadvantages of conventional TEM processing which utilizes silicone rubber mandrels include the fact that rubber tooling doe not have sufficient long term stability. That is, during repeated thermal pressure cycling, the rubber mandrels can experience a permanent compression set, which limits their life. Provisions must also be made in the part being manufactured to remove the mandrels after curing. This can compromise the shape or function of the part. Another disadvantage of the conventional TEM process is that a resulting hollow part of the object may lack sufficient structural strength. Still another disadvantage of rubber tooling is that if the rubber tooling is improperly sized, or if the cure cycle temperatures are exceeded, rubber tooling is capable of generating pressures sufficiently high to deform the tooling or ruin the mandrel. Yet another disadvantage of conventional TEM processing is that since the mandrels provide an interior mold surface, any imperfections in the rubber such as cuts, gouges, and the like will be transmitted to the part.
Several techniques are known for manufacturing paddles, which are useful in rowing, canoeing and the like, that are light in weight but yet structurally strong. Virtually all of these paddles are made either of wood, or they have an aluminum or fiberglass shaft with a blade of compression molded laminate or injection molded plastic. These paddles typically weight between 1 to 2.5 pounds. In another method of light weight paddle construction as described in U.S. Pat. No. 4,061,106, a core material such as balsa wood is cut to the shape of a paddle blade and then the blade is formed to the paddle shaft. Thereafter, the core is coated with a first layer of resin which waterproofs the wood and provides the required strength for the paddle. After the applied resin layer dries, the paddle blade is coated with a second layer of resin and then reinforcing fibers are laid onto the surface of the still wet resin coating. Upon drying of the resin coating, a complete paddle is obtained. While this method of paddle construction provides paddles of lighter weight construction than conventional paddles, the process is till relatively complex and involves a number of operational steps. A need therefore continues to exist for a relatively simpler way of manufacturing light weight paddles, as well as other structures, which involve fewer manufacturing steps.